Extracted from LBI-38379C
Maintenance Manual for the M-PA
403-512 MHz Portable FM Radio

Field RF Retuning Procedures

These field RF retuning procedures will be necessary if the factory tuned 20 - 22 MHz operating band does not meet needed requirements. For example, the factory tuned 450 - 470 MHz band can be retuned to cover the 440 - 460 MHz band. Factory tuned RF bands are listed in Table 1 of this manual and on the specification page. Synthesizer and receiver circuits must be retuned when the operating band is moved. Note that if the radio is retuned, the frequency range limits of the RF board cannot be exceeded. Perform the procedures in the order presented.


  1. Program the radio with the low, middle and high-side retune channels in the desired operating frequency band. The low and high-side channels should be 20 - 22 MHz apart and the middle channel should be centered between the two. Do not exceed the frequency range limits for the RF board.
  2. Separate the Front and Rear Cover Assemblies and connect the RF/Logic Extender cable between the RF Board and the Control Board.


  1. Set the radio to the middle retune channel. Monitor the VCO tuning voltage at TP1 (A5 pin 3) with a dc volt-meter. Adjust the RX VCO coil for a reading of 2.5 Vdc. See figure 2.
  2. Set the radio to the lowest retune channel. The monitored voltage should be greated than 1.0 Vdc.
  3. Set the radio to the highest retune channel. The monitored voltage should be less than 4.0 Vdc.
  4. If the lowest and highest retune channels puch the VCO tuning voltage mesaured at TP1 outside of the specified limits, slightly readjust the RX VCO coil to center the low and high retune channel tuning voltages within or around this 1.0 - 4.0 Vdc window.
  5. Repeat steps 1-4 for the TX VCO coil with the following exceptions: The radio must be keyed when adjustments are made to the TX VCO coil. Also, connect and appropriate RF load to the antenna before keying.
  6. Turn the radio off and connect the PC Programmer to the radio. Power the radio back up. Using the PC Programmer, set the modulation tracking Data value to OD (hex).
  7. Set the radio to the middle retune channel.
  8. Couple a modulation analyzer (with a monitoring oscilloscope) to the RF output of the radio. Set the modulation analyzer’s filters as follows: No high-pass filters and a 20 kHz low-pass filter. The modulation analyzer should have an almost dc response. If it does not, an alternative is to use a second M-PA monitoring the transmit frequency and monitor the discriminator output from the RF Board with a dc coupled oscilloscope. The monitoring point is J101 pin 4.
  9. Apply a 110 mV rms, 1 kHz tone to the TQ-3370’s modulation input. Key the radio from the TQ-3370 and adjust R19 on the RF Board for a deviation of +/- 4.3 kHz.
  10. Modify the RF/Logic Extender cable so a low-frequency signal can be injected directly into the RF Board. This modulation procedure is outlined in the TRANSMITTER ALIGNMENT of this manual.
  11. Using a 100uF (or greater) capacitor, couple 20Hz, 1.0 Vp-p square wave signal into the TX AUDIO (J102 pin 1).
  12. Key the radio and monitor the demodulated output from the modulation analyzer. Adjust R18 (modulation balance pot) for a good square wave response.


Network Analyzer Method

  1. Remove the solder mask on the solder side of the RF Board at the thin run between the output of FL4 and the input of A6 (A6 pin 1). Cut this run.
  2. Solder the Center conductor of a small 50-ohm coax cable to FL4’s output at the cut run, Solder the coax shield near FL4.
  3. Terminate the radio’s antenna with a 50-ohm load.
  4. Connect the other end of the coax to the network analyzer and tune FL4 (all three sections) for best return loss of the desired receive freqneucy range.
  5. Un-solder the coax cable from the output of FL4 and solder it to the input of A6. Ground the shield near A6.
  6. With the network analyzer’s output at -30 dBm or lower, adjust FL1 (all three sections) for best return loss over the desired receive frequency range. The receiver must be powered-up during this adjustment.
  7. With a small jumper, reconnect the output of FL4 to the input of A6. Test the radio over the new frequency range.

Spectrum Analyzer And Best Quieting Methods

This procedure involves retuning the receiver’s helical filters at the low, middle and high-side frequencies. The same frequencies used to retune the VCO can be used.

The preferred method is to connect a small 50-ohm coax to the output of IF crystal filter FL2 to ground, and connect this cable to a spectrum analyzer tuned to 45 MHz and set to 2 dB/division.

A less desirable alternative using the same tuning sequence can be achieved by varying the signal generator’s level and tuning for the best quieting (SINAD). No coax cable connectins are required using this procedure.

  1. Remove the input and output (outer) runing cores from FL1 and FL4 (four cores total).
  2. Set the radio and signal generator to the low-side receive frequency.
  3. Set the radio and signal generator’s level to -30 dBm. If tuning by quieting, set the generator’s level for a high receiver noise level.
  4. Alternative tune the center cores of FL1 and FL4 for maximum IF signal. If tuning by quieting, tune for sensitivity and reduce the generator’s level as the receiver becomes more sensitive.
  5. Set the radio and signal generator to the high-side receive frequency.
  6. Reinstall the input tuning cores into FL1 and FL4. Alternately adjust these cores for maximum signal or best quieting.
  7. Set the radio and signal generator to the middle receive frequency,
  8. Reinstall the output tuning cores into FL1 and FL4. Alternately adjust these cores for maximum signal or best quieting.
  9. Under certain circumstances, best tuning may be achieved when certan tuning cores are not reinstalled. This is acceptable. Also any excessively loose core should be secured with a drop of adhesive.

TABLE 1 - Test Frequencies (Mhz)

    RF Board     Freq. 1    Freq. 2    Freq. 3
    19D902282G1    403.025          413.025        422.975
    19D902282G6    450.125         460.025         469.975
    19D902282G3    470.025         481.025         491.975
    19D902282G8    492.025         503.025         513.975

TABLE 2 - Frequency Error Specifications

    RF Board              Frequency Split     Maximum Error
    19D902282G1            403-440 MHz              +/- 248 Hz
    19D902282G6           440-470 MHz             +/- 276 Hz
    19D902282G3            470-492 MHz              +/- 288 Hz
    19D902282G8            492-514 MHz              +/- 302 Hz

TABLE 3 - Tracking Data Parameters with Factory Settings

    Parameter Factory Setting
    High Power 5.0 - 5.2 watts
    Low Power 4.2 - 4.4 watts
    Modulation 4.2 - 4.4 kHz
    Squelch Opening 8 - 10 dB SINAD

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